1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and fabrication method thereof, and more particularly, to a liquid crystal display and fabrication method thereof in which both layers of organic insulating layer and inorganic insulating layer are used as a passivation film to allow a storage capacitor to have a large storage capacitance, thereby improving the display quality of displayed images.
2. Description of the Related Art
As information technology has advanced and society has become more and more dependent on display of information from various sources. Thus, there is a constant need for a flat panel display with superior characteristics such as slimness, lightweight and low power consumption. Among the various flat panel displays in existence, liquid crystal displays (LCDs) are superior in resolution, color display, display quality and the like. It is thus no wonder that LCDs are actively being employed in notebook computers and desktop computers.
In general, an LCD is formed using two substrates each having an electrode formed on an inner surface thereof. The substrates are arranged to face with each other, and liquid crystal material is injected into a space between the two substrates. In such LCDs, images are displayed by applying voltages to the substrates to generate an electric field, move the liquid crystal molecules of the liquid crystal layer selectively and thus change transmittance of light.
On an inner surface of the lower substrate of the LCD, a plurality of thin film transistors are arranged in a matrix configuration. On an inner surface of the upper substrate of the LCD, a common electrode is formed.
Pixel electrodes of the lower substrate and the common electrode of the upper substrate constitute liquid crystal capacitors. In each of the liquid crystal capacitors, a voltage applied to the liquid crystal capacitor is not held until the next signal is inputted but leaks away and disappears. To this end, the storage capacitor is formed to stabilize gray scale display and decrease flicker and residual image as well as to maintain the applied voltage.
The storage capacitor can be formed by two methods. In one method, a separate storage capacitor electrode is formed and is connected with the common electrode. In another method, a portion of the (n-1)-th gate line is used as the storage capacitor electrode of the n-th pixel.
The former type of storage capacitor is generally called ‘storage on common’ type or ‘independent storage capacitor’ type while the latter type of storage capacitor is generally called ‘storage on gate’ type or ‘previous gate’ type.
Hence, an LCD employing a storage capacitor formed by the storage on common type will be described below with reference to FIGS. 1 and 2. FIG. 1 is a plan view schematically showing one pixel in an LCD employing a storage capacitor formed by the conventional storage on common type and FIG. 2 is a sectional view taken along the line I-I′ of FIG. 1.
As shown in FIG. 1, on a lower substrate of an LCD, a gate line 11 and a data line 12 are formed to cross with each other. At a cross point of the gate line 11 and the data line 12, a thin film transistor 14 including gate electrode, source electrode and drain electrode are formed. At a cross point of the gate line 11 and the data line 12, a thin film transistor 14 each including gate electrode, source electrode and drain electrode are formed. At a pixel region defined by the pair of gate lines 11 and the pair of data lines 12 that cross with each other, a pixel electrode 15 contacting with the drain electrode is formed.
At a center portion of the pixel region, a storage capacitor lower electrode 13 is provided in a direction parallel with the gate line 11. The storage capacitor lower electrode 13 and the pixel electrode 15 form a storage capacitor. In other words, the pixel electrode 15 serves as an upper electrode of the storage capacitor.
Hereinbelow, structure of a storage capacitor formed on lower substrate of an LCD will be described with reference to FIG. 2.
In other words, as shown in FIG. 2, the storage capacitor lower electrode 13 formed on the substrate 21 and the pixel electrode formed above the storage capacitor lower electrode 13 form a storage capacitor. Here, the storage capacitor lower electrode 13 is formed of the same material as the gate electrode during the process of forming the gate electrode.
On the storage capacitor lower electrode 13, a gate insulating layer 23 and a passivation film 25 are sequentially formed. On the passivation film 25, the pixel electrode 15 is formed. The passivation film is inorganic insulator.
Detailed fabrication method of the aforementioned LCD will be omitted since it is widely known to those skilled to the art.
In the storage capacitance type LCD having the aforementioned structure, the storage capacitor is formed at pixel region and uses a separate storage electrode line as shown in FIGS. 1 and 2.
Either a common voltage applied to the lower substrate of the LCD or a separate storage voltage can be used as the storage voltage supplied to the storage electrode line. As the voltage drop (ΔVp) in an LC capacitor decreases as the storage capacitance of the storage capacitor increases, it becomes possible to display an image having a better picture quality with a larger storage capacitance.
Hence, one object in fabricating LCDs is to increase the capacitance of the storage capacitor as much as possible. However, other factors may balance the need for increasing the capacitance of the storage capacitor. For example, an LCD having a ferroelectric liquid crystal needs a large capacitance or a high resolution LCD having a high pixel density has a high area ratio of the storage capacitor to the pixel area, which decreases the aperture ratio of the pixel.
As one method to overcome this drawback, an organic insulator is used as the passivation film to increase the aperture ratio of the pixel. Unfortunately, the use organic insulator in and of itself is not enough at least because of the lower dielectric constant, thereby severely impacting the quality of displayed image. In an LCD in which the organic insulating layer is used as the passivation film, an inorganic insulating layer is also used along with the organic insulating layer, so that the passivation film is relatively thick. However, since the organic insulating layer has a dielectric constant lower than that of the inorganic insulating layer, the storage capacitance of the organic insulating layer is smaller than that of a comparably sized inorganic insulating layer.
Thus, since the storage capacitance of the LCD having a passivation film containing both an organic insulating layer and an inorganic insulating layer is smaller than that having only an inorganic insulating layer of the same size as the combination, the quality of displayed image is decreased relative to an LCD in which the passivation film contains only the inorganic insulating layer. Such a decrease, while less than that of an LCD in which the passivation film contains only the organic layer, remains unacceptable because of the need to increase the display quality while shrinking the display size.